Cost optimization of a solar-powered electrodialysis desalination system

Abstract

With their autonomous operation and low environmental impact, solar photovoltaics (PV) are an attractive power source for off-grid systems. One application of PV is for powering village-scale desalination systems, which are needed in regions with a saline drinking water supply and an unreliable electric grid. However, the intermittent and non-dispatchable nature of solar energy is not well-suited to conventional loads that are designed to operate off of a steady electrical grid, so it is important to design and optimize PV-powered systems such that they are persistent, reliable, predictable, and low-cost. In this thesis, I present a solar photovoltaic-powered electrodialysis reversal (PV-EDR) model, and use it to design a steady voltage and pumping EDR system composed of current off-the- shelf parts for Chelluru, a village near Hyderabad, India. I investigate flexible operation and load sizing as design approaches for low-cost PV-powered systems, and apply these concepts to a theoretical reference system and the PV-EDR system. I also present the results of a 7-day field test of the PV-EDR system in Chelluru. Through a sensitivity analysis performed with the PV-EDR model, I found that easing the output reliability constraint for the PV-EDR system from 100% to 98% reduced the system capital cost by 5.7%, indicating that usage of alternative water supplies during brief and infrequent periods of low sunshine could be a cost-effective way of supplementing PV-EDR if constant water production is required year-round. I found that the capital cost of the PV-EDR system was highly sensitive to the cost of the PV-EDR membranes, and foreseeable membrane cost reductions of 87% could reduce the cost of the total system by 50%. This observation was reaffirmed through an analysis of the effect of flexible operation and load sizing for PV-powered systems, which revealed that if the electrical load can be designed to operate primarily during the sunny hours of the day (as would be the case for a larger EDR unit enabled by inexpensive membranes), the PV and batteries could be downsized compared to a system that operates through the night. The PV-EDR model presented in this thesis was found to predict the operation of the installed system within 13% for the 7-day village test. This model can be adapted to other PVpowered systems to aid in design and cost optimization, and its accuracy will be further improved through additional testing and improved PV and battery device models. The flexible operation and load sizing design approaches detailed in this thesis will be useful for informing the design of any PV-powered system with accumulable output.